Over the years, the advantages of cryogenic cooling in being able to produce lower temperatures than conventionally available mechanical freezing apparatus has become apparent and has been found to be efficient for commercial applications. Cryogenic fast-freezers have proved to be of significant benefit to users desiring relatively low-temperature environments, for example, in the region of -30.degree. F. or -40.degree. F., and examples of such cryogenic freezing units are set forth in U.S. Pat. Nos. 3,660,985, 3,672,181, 3,754,407 and 3,815,377. Various of the apparatus illustrated in these patents were particularly designed for use with carbon dioxide, which offers particular advantages when cooling or freezing using temperatures in the aforementioned range.
U.S. Pat. No. 4,127,008, issued Nov. 28, 1978 to Lewis Tyree, Jr., shows a variety of cryogenic cooling apparatus including some which are designed to employ a closed chamber in which a static or stationary load can be placed, for example, by rolling in a wheeled cart or rack carrying the product disposed on vertically spaced shelves. The cooling is accomplished in the illustrated devices solely by the use of cryogen which was expanded in a heat-exchanger and recovered, requiring slightly more sophisticated equipment. It is felt that there is a commercial market for somewhat simpler devices which so efficiently utilize cryogen to cool or freeze a load within a compartment that expenditure of the cryogen can be permitted while remaining cost effective.